FIG. 1A illustrates a prior art dc-dc converter circuit 100. In such a buck converter circuit 100 there are two switch devices, which are known as the control (or high side) FET 104 and the sync (or low side) FET 105. As can be seen in the diagram, the control FET's source terminal is connected directly to the sync FET's drain terminal at node 102. A voltage source provides current at Vin 101. The node 102 between the two devices (known as the switching node) is also connected to the converter's output through an LC network comprising an inductor 106 and a capacitor 107. Vin 101 and Vout 108 are referenced to ground 103. A driver 109 provides control for switching the control FET 104 and sync FET 105 on and off.
Conventionally high-current (>˜1 amp) output converters use silicon MOSFET devices such as depicted in FIG. 1A for the switches 104 and 105. These MOSFETs are vertical devices; that is the current flows through the bulk between the drain contact (which is conventionally located on the top surface of the die) and the source contact on the opposite surface when the switch 104 and/or 105 is in its on state. Because of this vertical current flow, the control FET device 104 and the sync FET device 105 are on separate die. This then means that a bottom surface of the control FET die 104 must be electrically connected to a top surface of the sync FET die 105, and, at the same time, connected to the inductor 106 that goes to the converter output 108 across a load R. In addition, since heat is generated in the bulk of the device, both surfaces are normally heat sunk to minimize the temperature rise. These requirements lead to a relatively complex packaging problem.